Bone marrow NR4A expression is not a dominant factor in the development of atherosclerosis or macrophage polarization in mice

Long COVID Definition, Symptoms, Risk Factors, Epidemiology and Autoimmunity: A Narrative Review

The virus called SARS-CoV-2 emerged in 2019 and quickly spread worldwide, causing COVID-19. It has greatly impacted on everyday life, healthcare systems, and the global economy. In order to save as many lives as possible, precautions such as social distancing, quarantine, and testing policies were implemented, and effective vaccines were developed. A growing amount of data collected worldwide allowed the characterization of this new disease, which turned out to be more complex than other common respiratory tract infections. An increasing number of convalescents presented with a variety of nonspecific symptoms emerging after the acute infection. This possible new global health problem was identified and labelled as long COVID. Since then, a great effort has been made by clinicians and the scientific community to understand the underlying mechanisms and to develop preventive measures and effective treatment. The role of autoimmunity induced by SARS-CoV-2 infection in the development of long COVID is discussed in this review. We aim to deliver a description of several conditions with an autoimmune background observed in COVID-19 convalescents, including Guillain-Barré syndrome, antiphospholipid syndrome and related thrombosis, and Kawasaki disease highlighting a relationship between SARS-CoV-2 infection and the development of autoimmunity. However, further studies are required to determine its true clinical significance.

Dexmedetomidine attenuates inflammation and organ injury partially by upregulating Nur77 in sepsis

PURPOSE

The aim of this study was to investigate the effect of dexmedetomidine (Dex) on inflammation and organ injury in sepsis, as well as the potential relationship between Dex and nuclear receptor 77 (Nur77).

METHODS

We investigated the effects of dexmedetomidine on lipopolysaccharide (LPS)-induced inflammation in RAW264.7 cells and organ injury in the cecal ligation and puncture (CLP) mouse model. Additionally, we examined the relationship between dexmedetomidine and Nur77. The expression levels of Nur77 in RAW264.7 cells were analyzed under various types of stimulation using quantitative reverse transcription polymerase chain reaction and western blot analysis. Inflammatory cytokine levels in the cells were evaluated using enzyme-linked immunoassay. Organ injuries were assessed by examining tissue histology and pathology of the lung, liver, and kidney.

RESULTS

Dexmedetomidine increased the expression of Nur77 and IL-10, and downregulated inflammatory cytokines (IL-1β and TNF-α) in LPS-treated RAW264.7 cells. The effect of dexmedetomidine on inhibiting inflammation in LPS-treated RAW264.7 cells was promoted by overexpressing Nur77, while it was reversed by downregulating Nur77. Additionally, dexmedetomidine promoted the expression of Nur77 in the lung and CLP-induced pathological changes in the lung, liver, and kidney. Activation of Nur77 with the agonist Cytosporone B (CsnB) significantly suppressed the production of IL-1β and TNF-α in LPS-treated RAW264.7 cells. In contrast, knockdown of Nur77 augmented IL-1β and TNF-α production in LPS-treated RAW264.7 cells.

CONCLUSION

Dexmedetomidine can attenuate inflammation and organ injury, at least partially, via upregulating Nur77 in sepsis.

Nur77 influences immunometabolism to regulate the release of proinflammatory cytokines and the formation of lipid bodies during Mycobacterium tuberculosis infection of macrophages

Infection of macrophages with Mycobacterium tuberculosis induces innate immune responses designed to clear the invading bacterium. However, bacteria often survive within the intracellular environment by exploiting these responses triggered by macrophages. Here, the role of the orphan nuclear receptor Nur77 (Nr4a1) in regulating the response of macrophages infected with M. tuberculosis (Mtb) has been delineated. Nur77 is induced early during infection, regulates metabolism by binding directly at the promoter of the TCA cycle enzyme, isocitrate dehydrogenase 2 (IDH2), to act as its repressor, and shifts the balance from a proinflammatory to an anti-inflammatory phenotype. Depletion of Nur77 increased transcription of IDH2 and, consequently, the levels of intracellular succinate, leading to enhanced levels of the proinflammatory cytokine IL-1β. Further, Nur77 inhibited the production of antibacterial nitric oxide and IL-1β in a succinate dehydrogenase (SDH)-dependent manner, suggesting that its induction favors bacterial survival by suppressing bactericidal responses. Indeed, depletion of Nur77 inhibited the intracellular survival of Mtb. On the other hand, depletion of Nur77 enhanced lipid body formation, suggesting that the fall in Nur77 levels as infection progresses likely favors foamy macrophage formation and long-term survival of Mtb in the host milieu.

Nur77 and PPARγ regulate transcription and polarization in distinct subsets of M2-like reparative macrophages during regenerative inflammation

Macrophage polarization is a process whereby macrophages develop a specific phenotype and functional response to different pathophysiological stimuli and tissue environments. In general, two main macrophage phenotypes have been identified: inflammatory (M1) and alternatively activated (M2) macrophages characterized specifically by IL-1β and IL-10 production, respectively. In the cardiotoxin-induced skeletal muscle injury model bone marrow-derived macrophages (BMDMs) play the central role in regulating tissue repair. Bone marrow-derived monocytes arriving at the site of injury differentiate first to M1 BMDMs that clear cell debris and trigger proliferation and differentiation of the muscle stem cells, while during the process of efferocytosis they change their phenotype to M2 to drive resolution of inflammation and tissue repair. The M2 population is formed from at least three distinct subsets: antigen presenting, resolution-related and growth factor producing macrophages, the latest ones expressing the transcription factor PPARγ. Nuclear receptor subfamily 4 group A member 1 (NR4A1; also termed Nur77) transcription factor is expressed as an early response gene, and has been shown to suppress the expression of pro-inflammatory genes during efferocytosis. Here we demonstrate that (1) Nur77 null BMDMs are characterized by elevated expression of PPARγ resulting in enhanced efferocytosis capacity; (2) Nur77 and PPARγ regulate transcription in different subsets of M2 skeletal muscle macrophages during muscle repair; (3) the loss of Nur77 prolongs M1 polarization characterized by increased and prolonged production of IL-1β by the resolution-related macrophages normally expressing Nur77; whereas, in contrast, (4) it promotes M2 polarization detected via the increased number of IL-10 producing CD206⁺ macrophages generated from the PPARγ-expressing subset.

Nur77 Deficiency Exacerbates Macrophage NLRP3 Inflammasome-Mediated Inflammation and Accelerates Atherosclerosis

Purpose

Activation of NLR (nucleotide-binding and leucine-rich repeat immune receptor) family pyrin domain containing 3 (NLRP3) inflammasome mediating interleukin- (IL-) 1β secretion has emerged as an important component of inflammatory processes in atherogenesis. The nuclear receptor Nur77 is highly expressed in human atherosclerotic lesions; however, its functional role in macrophage NLRP3 inflammasome activation has not yet been clarified. Methods, Materials, and Results. Eight-week-old apolipoprotein E (ApoE)−/− and ApoE−/− Nur77−/− mice that were fed a Western diet underwent partial ligation of the left common carotid artery (LCCA) and left renal artery (LRA) to induce atherogenesis. Four weeks later, severe plaque burden associated with increased lipid deposition, reduced smooth muscle cells, macrophage infiltration, and decreased collagen expression was identified in ApoE−/− Nur77−/− mice compared with those in ApoE−/− mice. ApoE−/− Nur77−/− mice showed increased macrophage inflammatory responses in carotid atherosclerotic lesions. In vitro studies demonstrated that oxidized low-density lipoprotein cholesterol (ox-LDL) increased the release of lactate dehydrogenase (LDH) and upregulated the expressions of cleaved caspase-1, cleaved IL-1β and gasdermin D (GSMD) in WT peritoneal macrophages (PMs) in a NLRP3-dependent manner. Nur77−/− PMs exhibited a further increased level of NLRP3 inflammasome-mediated inflammation under ox-LDL treatment compared with WT PMs. Mechanistically, Nur77 could bind to the promoter of NLRP3 and inhibit its transcriptional activity.

Conclusions

This study demonstrated that Nur77 deletion promotes atherogenesis by exacerbating NLRP3 inflammasome-mediated inflammation.

Elevated granulocyte-colony stimulating factor and hematopoietic stem cell mobilization in Niemann-Pick type C1 disease

Niemann-Pick type C1 (NPC1) disease is a progressive lysosomal storage disorder caused by mutations of the NPC1 gene. While neurodegeneration is the most severe symptom, a large proportion of NPC1 patients also present with splenomegaly, which has been attributed to cholesterol and glycosphingolipid accumulation in late endosomes and lysosomes. However, recent data also reveal an increase in the inflammatory monocyte subset in the Npc1nih mouse model expressing an Npc1 null allele. We evaluated the contribution of hematopoietic cells to splenomegaly in NPC1 disease under conditions of hypercholesterolemia. We transplanted Npc1nih (Npc1 null mutation) or Npc1wt bone marrow (BM) into Ldlr-/- mice and fed these mice a cholesterol-rich Western-type diet. At 9 weeks after BM transplant, on a chow diet, the Npc1 null mutation increased plasma granulocyte-colony stimulating factor (G-CSF) by 2-fold and caused mild neutrophilia. At 18 weeks after BM transplant, including 9 weeks of Western-type diet feeding, the Npc1 mutation increased G-csf mRNA levels by ∼5-fold in splenic monocytes/macrophages accompanied by a ∼4-fold increase in splenic neutrophils compared with controls. We also observed ∼5-fold increased long-term and short-term hematopoietic stem cells (HSCs) in the spleen, and a ∼30-75% decrease of these populations in BM, reflecting HSC mobilization, presumably downstream of elevated G-CSF. In line with these data, four patients with NPC1 disease showed higher plasma G-CSF compared with age-matched and gender-matched healthy controls. In conclusion, we show elevated G-CSF levels and HSC mobilization in the setting of an Npc1 null mutation and propose that this contributes to splenomegaly in patients with NPC1 disease.

Role of NR4A family members in myeloid cells and leukemia

The myeloid cellular compartment comprises monocytes, dendritic cells (DCs), macrophages and granulocytes. As diverse as this group of cells may be, they are all an important part of the innate immune system and are therefore linked by the necessity to be acutely sensitive to their environment and to rapidly and appropriately respond to any changes that may occur. The nuclear orphan receptors NR4A1, NR4A2 and NR4A3 are encoded by immediate early genes as their expression is rapidly induced in response to various signals. It is perhaps because of this characteristic that this family of transcription factors has many known roles in myeloid cells. In this review, we will regroup and discuss the diverse roles NR4As have in different myeloid cell subsets, including in differentiation, migration, activation, and metabolism. We will also highlight the importance these molecules have in the development of myeloid leukemia.

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NR4A1-3 have important roles in the different cells of the myeloid compartment.

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These orphan receptors homeostasis, differentiation, and activation.

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NR4A family is important in suppressing the development of myeloid leukemias.

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NR4As have been linked to several diseases and could be pharmacological targets.

NR4A3: A Key Nuclear Receptor in Vascular Biology, Cardiovascular Remodeling, and Beyond

The mechanisms committed in the activation and response of vascular and inflammatory immune cells play a major role in tissue remodeling in cardiovascular diseases (CVDs) such as atherosclerosis, pulmonary arterial hypertension, and abdominal aortic aneurysm. Cardiovascular remodeling entails interrelated cellular processes (proliferation, survival/apoptosis, inflammation, extracellular matrix (ECM) synthesis/degradation, redox homeostasis, etc.) coordinately regulated by a reduced number of transcription factors. Nuclear receptors of the subfamily 4 group A (NR4A) have recently emerged as key master genes in multiple cellular processes and vital functions of different organs, and have been involved in a variety of high-incidence human pathologies including atherosclerosis and other CVDs. This paper reviews the major findings involving NR4A3 (Neuron-derived Orphan Receptor 1, NOR-1) in the cardiovascular remodeling operating in these diseases.

Nuclear receptor Nur77: its role in chronic inflammatory diseases

Nur77 is a nuclear receptor that has been implicated as a regulator of inflammatory disease. The expression of Nur77 increases upon stimulation of immune cells and is differentially expressed in chronically inflamed organs in human and experimental models. Furthermore, in a variety of animal models dedicated to study inflammatory diseases, changes in Nur77 expression alter disease outcome. The available studies comprise a wealth of information on the function of Nur77 in diverse cell types and tissues. Negative cross-talk of Nur77 with the NFκB signaling complex is an example of Nur77 effector function. An alternative mechanism of action has been established, involving Nur77-mediated modulation of metabolism in macrophages as well as in T cells. In this review, we summarize our current knowledge on the role of Nur77 in atherosclerosis, inflammatory bowel disease, multiple sclerosis, rheumatoid arthritis, and sepsis. Detailed insight in the control of inflammatory responses will be essential in order to advance Nur77-targeted therapeutic interventions in inflammatory disease.

Targeting Orphan Nuclear Receptors NR4As for Energy Homeostasis and Diabetes

Orphan nuclear receptors are important members of the nuclear receptor family and may regulate cell proliferation, metabolism, differentiation, and apoptosis. NR4As, a subfamily of orphan nuclear receptors, have been reported to play key roles in carbohydrate and lipid metabolism and energy homeostasis. Popularity of obesity has resulted in a series of metabolic diseases such as diabetes and its complications. While imbalance of energy intake and expenditure is the main cause of obesity, the concrete mechanism of obesity has not been fully understood. It has been reported that NR4As have significant regulatory effects on energy homeostasis and diabetes and are expected to become new targets for discovering drugs for metabolic syndrome. A number of studies have demonstrated that abnormalities in metabolism induced by altered levels of NR4As may contribute to numerous diseases, such as chronic inflammation, tumorigenesis, diabetes and its complications, atherosclerosis, and other cardiovascular diseases. However, systematic reviews focusing on the roles of NR4As in mediating energy homeostasis and diabetes remain limited. Therefore, this article reviews the structure and regulation of NR4As and their critical function in energy homeostasis and diabetes, as well as small molecules that may regulate NR4As. Our work is aimed at providing valuable support for the research and development of drugs targeting NR4As for the treatment of obesity and related metabolic diseases.

NR4A1 Deletion in Marginal Zone B Cells Exacerbates Atherosclerosis in Mice-Brief Report

Supplemental Digital Content is available in the text.

NR4A orphan receptors have been well studied in vascular and myeloid cells where they play important roles in the regulation of inflammation in atherosclerosis. NR4A1 (nerve growth factor IB) is among the most highly induced transcription factors in B cells following BCR (B-cell receptor) stimulation. Given that B cells substantially contribute to the development of atherosclerosis, we examined whether NR4A1 regulates B-cell function during atherogenesis.

Nuclear receptors in abdominal aortic aneurysms

Abdominal aortic aneurysms (AAA) pose a considerable health burden and at present are only managed surgically since there is no proven pharmacotherapy that will retard their expansion or reduce the incidence of fatal rupture. This pathology shares several pathophysiological mechanisms with atherosclerosis, such as macrophage infiltration, inflammation, and degradation of extracellular matrix. Therefore, therapeutic targets proven effective in the treatment of atherosclerosis could also be considered for treatment of AAA. Different members of the nuclear receptor (NR) superfamily have been extensively studied as potential targets in the treatment of cardiovascular disease (CVD) and therefore might also be suited for AAA treatment. In this context, this review summarizes the role of different NRs in CVD, mostly atherosclerosis, and discusses in detail the current knowledge of their implications in AAA. From this overview it becomes apparent that NRs that were attributed a beneficial or adverse role in CVD have similar roles in AAA. Together, this overview provides compelling evidence to consider several NRs as attractive targets for future treatment of AAA.

Immunoregulatory Functions of Nuclear Receptors: Mechanisms and Therapeutic Implications

Members of the nuclear receptor superfamily serve as master regulators in signaling by either positively or negatively regulating gene expression. Accumulating evidence has suggested that nuclear receptors are actively involved in immune responses, with specific roles in different immune cell compartments that contribute to both normal function and to disease development. The druggable properties of nuclear receptors have made them ideal modulatory therapeutic targets. Here, we revisit nuclear receptor biology, summarize recent advances in our understanding of the immunological functions of nuclear receptors, describe cell-type-specific roles and specific nuclear receptors in disease pathogenesis, and explore their potential as novel therapeutic targets. These nuclear receptor-dependent alterations in the immune system are amenable to pharmacological manipulation and suggest novel therapeutic strategies.

Nuclear Receptor Nur77 Limits the Macrophage Inflammatory Response through Transcriptional Reprogramming of Mitochondrial Metabolism

Activation of macrophages by inflammatory stimuli induces reprogramming of mitochondrial metabolism to support the production of pro-inflammatory cytokines and nitric oxide. Hallmarks of this metabolic rewiring are downregulation of α-ketoglutarate formation by isocitrate dehydrogenase (IDH) and accumulation of glutamine-derived succinate, which enhances the inflammatory response via the activity of succinate dehydrogenase (SDH). Here, we identify the nuclear receptor Nur77 (Nr4a1) as a key upstream transcriptional regulator of this pro-inflammatory metabolic switch in macrophages. Nur77-deficient macrophages fail to downregulate IDH expression and accumulate higher levels of succinate and other TCA cycle-derived metabolites in response to inflammatory stimulation in a glutamine-independent manner. Consequently, these macrophages produce more nitric oxide and pro-inflammatory cytokines in an SDH-dependent manner. In vivo, bone marrow Nur77 deficiency exacerbates atherosclerosis development and leads to increased circulating succinate levels. In summary, Nur77 induces an anti-inflammatory metabolic state in macrophages that protects against chronic inflammatory diseases such as atherosclerosis.

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Genome-wide profiling indicates that Nur77 regulates macrophage mitochondrial metabolism

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Nur77 inhibits IDH expression and TCA cycle activity in inflammatory macrophages

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Nur77-deficient macrophages produce more nitric oxide and cytokines via SDH

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Nur77 deficiency increases circulating succinate levels and atherosclerosis in vivo

Overexpression of myeloid angiotensin-converting enzyme (ACE) reduces atherosclerosis

BACKGROUND

Macrophages are ubiquitous in all stages of atherosclerosis, exerting tremendous impact on lesion progression and plaque stability. Because macrophages in atherosclerotic plaques express angiotensin-converting enzyme (ACE), current dogma posits that local myeloid-mediated effects worsen the disease. In contrast, we previously reported that myeloid ACE overexpression augments macrophage resistance to various immune challenges, including tumors, bacterial infection and Alzheimer's plaque deposition. Here, we sought to assess the impact of myeloid ACE on atherosclerosis.

METHODS

A mouse model in which ACE is overexpressed in myelomonocytic lineage cells, called ACE10, was generated and sequentially crossed with ApoE-deficient mice to create ACE^10/10ApoE^-/- (ACE10/ApoE). Control mice were ACE^WT/WTApoE^-/- (WT/ApoE). Atherosclerosis was induced using an atherogenic diet alone, or in combination with unilateral nephrectomy plus deoxycorticosterone acetate (DOCA) salt for eight weeks.

RESULTS

With an atherogenic diet alone or in combination with DOCA, the ACE10/ApoE mice showed significantly less atherosclerotic plaques compared to their WT/ApoE counterparts (p < 0.01). When recipient ApoE^-/- mice were reconstituted with ACE^10/10 bone marrow, these mice showed significantly reduced lesion areas compared to recipients reconstituted with wild type bone marrow. Furthermore, transfer of ACE-deficient bone marrow had no impact on lesion area.

CONCLUSION

Our data indicate that while myeloid ACE may not be required for atherosclerosis, enhanced ACE expression paradoxically reduced disease progression.

A permethrin metabolite is associated with adaptive immune responses in Gulf War Illness

Gulf War Illness (GWI), affecting 30% of veterans from the 1991 Gulf War (GW), is a multi-symptom illness with features similar to those of patients with autoimmune diseases. The objective of the current work is to determine if exposure to GW-related pesticides, such as permethrin (PER), activates peripheral and central nervous system (CNS) adaptive immune responses. In the current study, we focused on a PER metabolite, 3-phenoxybenzoic acid (3-PBA), as this is a common metabolite previously shown to form adducts with endogenous proteins. We observed the presence of 3-PBA and 3-PBA modified lysine of protein peptides in the brain, blood and liver of pyridostigmine bromide (PB) and  PER (PB+PER) exposed mice at acute and chronic post-exposure timepoints. We tested whether 3-PBA-haptenated albumin (3-PBA-albumin) can activate immune cells since it is known that chemically haptenated proteins can stimulate immune responses. We detected autoantibodies against 3-PBA-albumin in plasma from PB + PER exposed mice and veterans with GWI at chronic post-exposure timepoints. We also observed that in vitro treatment of blood with 3-PBA-albumin resulted in the activation of B- and T-helper lymphocytes and that these immune cells were also increased in blood of PB + PER exposed mice and veterans with GWI. These immune changes corresponded with elevated levels of infiltrating monocytes in the brain and blood of PB + PER exposed mice which coincided with alterations in the markers of blood-brain barrier disruption, brain macrophages and neuroinflammation. These studies suggest that pesticide exposure associated with GWI may have resulted in the activation of the peripheral and CNS adaptive immune responses, possibly contributing to an autoimmune-type phenotype in veterans with GWI.

Monocytes, Macrophages, and Metabolic Disease in Atherosclerosis

Atherosclerotic cardiovascular disease (CVD) is a lipid-driven chronic inflammatory disease, in which macrophages are responsible for taking up these lipids and driving disease progression. Over the years, we and others have uncovered key pathways that regulate macrophage number/function and identified how metabolic disorders such as diabetes and obesity, which are common risk factors for CVD, exacerbate these pathways. This ultimately accelerates the progression of atherosclerosis and hinders atherosclerotic regression. In this review, we discuss the different types of macrophages, from monocyte-derived macrophages, local macrophage proliferation, to macrophage-like vascular smooth muscle cells, that contribute to atherosclerosis as well as myeloid-derived suppressor cells that may have anti-atherogenic effects. We will also discuss how diabetes and obesity influence plaque macrophage accumulation and monocyte production (myelopoiesis) to promote atherogenesis as well as an exciting therapeutic target, S100A8/A9, which mediates myelopoiesis in response to both diabetes and obesity, shown to be effective in reducing atherosclerosis in pre-clinical models of diabetes.

Nonclassical Monocytes in Health and Disease

Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14+CD16−in humans and Ly6Chiin mice), intermediate (CD14+CD16+in humans and Ly6C+Treml4+in mice), and nonclassical (CD14−CD16+in humans and Ly6Cloin mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.

MicroRNA-21 Deficiency Alters the Survival of Ly-6C lo Monocytes in ApoE −/− Mice and Reduces Early-Stage Atherosclerosis—Brief Report

Objective—

To determine the role of microRNA-21 (miR-21) on the homeostasis of monocyte subsets and on atherosclerosis development in ApoE −/− (apolipoprotein E) mice.

Approach and Results—

In ApoE −/− mice, miR-21 expression was increased in circulating Ly-6C lo nonclassical monocytes in comparison to Ly-6C hi monocytes. The absence of miR-21 significantly altered the survival and number of circulating Ly-6C lo nonclassical monocytes in ApoE −/− mice. In the early stages of atherosclerosis, the absence of miR-21 limited lesion development both in the aortic sinus (by almost 30%) and in the aorta (by almost 50%). This was associated with less monocyte availability in circulation and increased apoptosis of local macrophages in plaques. At later stages of atherosclerosis, lesion size in the aortic root was similar in ApoE −/− and ApoE −/− miR-21 −/− mice, but plaques showed a less stable phenotype (larger necrotic cores) in the latter. The loss of protection in advanced stages was most likely because of excessive inflammatory apoptosis related to an impairment of local efficient efferocytosis.

Conclusions—

Gene deletion of miR-21 in ApoE −/− mice alters Ly-6C lo nonclassical monocytes homeostasis and contribute to limit early-stage atherosclerosis.

Circulating CD14+CD16- classical monocytes do not associate with a vulnerable plaque phenotype, and do not predict secondary events in severe atherosclerotic patients

AIMS

Mouse studies have established distinct monocyte subtypes that participate in the process of atherosclerotic lesion formation. The pro-inflammatory Ly6C^high monocyte subtype actively contributes to murine plaque progression and destabilization. Also in humans, different peripheral monocyte subtypes have been identified, of which the CD14⁺CD16^- classical monocyte is suggested to display similar pro-atherosclerotic properties as the murine Ly6C^high subtype. We aimed to investigate if circulating CD14⁺CD16^- classical monocytes associate with characteristics of a vulnerable carotid atherosclerotic plaque and if they associate with the risk of secondary adverse manifestations of atherosclerotic disease.

METHODS AND RESULTS

We enrolled 175 carotid endarterectomy patients of the Athero-Express biobank in our study. Just prior to surgical procedure, blood was collected and peripheral blood mononuclear cells were isolated. Characterization of monocyte subsets was performed by flow cytometry. Plaque characteristics were semi-quantitatively scored for the presence of fat, collagen, intraplaque hemorrhage and calcification. Vessel density, smooth muscle cells and macrophages were assessed quantitatively on a continuous scale. All features of a vulnerable plaque phenotype, including low amounts of collagen and smooth muscle cells, and increased fat content, vessel density, intraplaque hemorrhage and plaque macrophages were not significantly associated with differential levels of peripheral classical CD14⁺CD16^- monocytes or other monocyte subsets. Using Cox regression models to evaluate the prognostic value of circulating monocyte subtypes, we found that total counts of peripheral monocytes, as well as CD14⁺CD16^- classical and other monocyte subtypes were not associated with the risk of secondary cardiovascular events during 3 years follow-up.

CONCLUSION

Circulating classical CD14⁺CD16^- monocytes do not associate with specific vulnerable plaque characteristics. In addition, they do not predict secondary adverse manifestations. This suggests that in patients with established carotid artery disease, the circulating monocytes do not reflect plaque characteristics and have no value in identifying patients at risk for future cardiovascular events.

Retinoid X Receptor Antagonists

Retinoid X receptor (RXR) antagonists are not only useful as chemical tools for biological research, but are also candidate drugs for the treatment of various diseases, including diabetes and allergies, although no RXR antagonist has yet been approved for clinical use. In this review, we present a brief overview of RXR structure, function, and target genes, and describe currently available RXR antagonists, their structural classification, and their evaluation, focusing on the latest research.

Insights From Pre-Clinical and Clinical Studies on the Role of Innate Inflammation in Atherosclerosis Regression

Atherosclerosis, the underlying cause of coronary artery (CAD) and other cardiovascular diseases, is initiated by macrophage-mediated immune responses to lipoprotein and cholesterol accumulation in artery walls, which result in the formation of plaques. Unlike at other sites of inflammation, the immune response becomes maladaptive and inflammation fails to resolve. The most common treatment for reducing the risk from atherosclerosis is low density lipoprotein cholesterol (LDL-C) lowering. Studies have shown, however, that while significant lowering of LDL-C reduces the risk of heart attacks to some degree, there is still residual risk for the majority of the population. We and others have observed “residual inflammatory risk” of atherosclerosis after plasma cholesterol lowering in pre-clinical studies, and that this phenomenon is clinically relevant has been dramatically reinforced by the recent Canakinumab Anti-inflammatory Thrombosis Outcomes Study (CANTOS) trial. This review will summarize the role of the innate immune system, specifically macrophages, in atherosclerosis progression and regression, as well as the pre-clinical and clinical models that have provided significant insights into molecular pathways involved in the resolution of plaque inflammation and plaque regression. Partnered with clinical studies that can be envisioned in the post-CANTOS period, including progress in developing targeted plaque therapies, we expect that pre-clinical studies advancing on the path summarized in this review, already revealing key mechanisms, will continue to be essential contributors to achieve the goals of dampening plaque inflammation and inducing its resolution in order to maximize the therapeutic benefits of conventional risk factor modifications, such as LDL-C lowering.

MT4-MMP deficiency increases patrolling monocyte recruitment to early lesions and accelerates atherosclerosis

Matrix metalloproteinases are involved in vascular remodeling. Little is known about their immune regulatory role in atherosclerosis. Here we show that mice deficient for MT4-MMP have increased adherence of macrophages to inflamed peritonea, and larger lipid deposits and macrophage burden in atherosclerotic plaques. We also demonstrate that MT4-MMP deficiency results in higher numbers of patrolling monocytes crawling and adhered to inflamed endothelia, and the accumulation of Mafb+ apoptosis inhibitor of macrophage (AIM)+ macrophages at incipient atherosclerotic lesions in mice. Functionally, MT4-MMP-null Mafb+AIM+ peritoneal macrophages express higher AIM and scavenger receptor CD36, are more resistant to apoptosis, and bind acLDL avidly, all of which contribute to atherosclerosis. CCR5 inhibition alleviates these effects by hindering the enhanced recruitment of MT4-MMP-null patrolling monocytes to early atherosclerotic lesions, thus blocking Mafb+AIM+ macrophage accumulation and atherosclerosis acceleration. Our results suggest that MT4-MMP targeting may constitute a novel strategy to boost patrolling monocyte activity in early inflammation.

Functional diversity of macrophages in vascular biology and disease

Atherosclerosis is a multifactorial chronic inflammatory disease and is largely responsible for cardiovascular disease, the most common cause of global mortality. The hallmark of atherogenesis is immune activation following lipid accumulation in the arterial wall. In particular, macrophages play a non-redundant role in both the progression and regression of inflammation in the atherosclerotic lesion. Macrophages are remarkably heterogeneous phagocytes that perform versatile functions in health and disease. Their functional diversity in vascular biology is only partially mapped. Targeting macrophages is often highlighted as a therapeutic approach for cancer, metabolic and inflammatory diseases. Future strategies for therapeutic intervention in atherosclerosis may benefit from attempts to reduce local proliferation of pro-inflammatory macrophage subsets or enhance resolution of inflammation. Thus, characterisation of macrophage subsets during atherosclerosis would empower clinical interventions. Therefore, it would be of fundamental importance to understand how pathological factors modulate macrophage activity in order to exploit their use in the treatment of atherosclerosis and other diseases.

Effects of dyslipidaemia on monocyte production and function in cardiovascular disease

Monocytes are heterogeneous effector cells involved in the maintenance and restoration of tissue integrity. Monocytes and macrophages are involved in cardiovascular disease progression, and are associated with the development of unstable atherosclerotic plaques. Hyperlipidaemia can accelerate cardiovascular disease progression. However, monocyte responses to hyperlipidaemia are poorly understood. In the past decade, accumulating data describe the relationship between the dynamic blood lipid environment and the heterogeneous circulating monocyte pool, which might have profound consequences for cardiovascular disease. In this Review, we explore the updated view of monocytes in cardiovascular disease and their relationship with macrophages in promoting the homeostatic and inflammatory responses related to atherosclerosis. We describe the different definitions of dyslipidaemia, highlight current theories on the ontogeny of monocyte heterogeneity, discuss how dyslipidaemia might alter monocyte production, and explore the mechanistic interface linking dyslipidaemia with monocyte effector functions, such as migration and the inflammatory response. Finally, we discuss the role of dietary and endogenous lipid species in mediating dyslipidaemic responses, and the role of these lipids in promoting the risk of cardiovascular disease through modulation of monocyte behaviour.

Map3k8 Modulates Monocyte State and Atherogenesis in ApoE-/- Mice

OBJECTIVE

Map3k8 (Cot/Tpl2) activates the MKK1/2-ERK1/2, MAPK pathway downstream from interleukin-1R, tumor necrosis factor-αR, NOD-2R (nucleotide-binding oligomerization domain-like 2R), adiponectinR, and Toll-like receptors. Map3k8 plays a key role in innate and adaptive immunity and influences inflammatory processes by modulating the functions of different cell types. However, its role in atherogenesis remains unknown. In this study, we analyzed the role of this kinase in this pathology.

APPROACH AND RESULTS

We show here that Map3k8 deficiency results in smaller numbers of Ly6C^highCD11c^low and Ly6C^lowCD11c^high monocytes in ApoE^-/- mice fed a high-fat diet (HFD). Map3k8^-/-ApoE^-/- monocytes displayed high rates of apoptosis and reduced amounts of Nr4a1, a transcription factor known to modulate apoptosis in Ly6C^lowCD11c^high monocytes. Map3k8^-/-ApoE^-/- splenocytes and macrophages showed irregular patterns of cytokine and chemokine expression. Map3k8 deficiency altered cell adhesion and migration in vivo and decreased CCR2 expression, a determinant chemokine receptor for monocyte mobilization, on circulating Ly6C^highCD11c^low monocytes. Map3k8^-/-ApoE^-/- mice fed an HFD showed decreased cellular infiltration in the atherosclerotic plaque, with low lipid content. Lesions had similar size after Map3k8^+/+ApoE^-/- bone marrow transplant into Map3k8^-/-ApoE^-/- and Map3k8^+/+ApoE^-/- mice fed an HFD, whereas smaller plaques were observed after the transplantation of bone marrow lacking both ApoE and Map3k8.

CONCLUSIONS

Map3k8 decreases apoptosis of monocytes and enhances CCR2 expression on Ly6C^highCD11c^low monocytes of ApoE^-/- mice fed an HFD. These findings explain the smaller aortic lesions in ApoE^-/- mice with Map3k8^-/-ApoE^-/- bone marrow cells fed an HFD, supporting further studies of Map3k8 as an antiatherosclerotic target.

Polarization of M2 macrophages requires Lamtor1 that integrates cytokine and amino-acid signals

Macrophages play crucial roles in host defence and tissue homoeostasis, processes in which both environmental stimuli and intracellularly generated metabolites influence activation of macrophages. Activated macrophages are classified into M1 and M2 macrophages. It remains unclear how intracellular nutrition sufficiency, especially for amino acid, influences on macrophage activation. Here we show that a lysosomal adaptor protein Lamtor1, which forms an amino-acid sensing complex with lysosomal vacuolar-type H⁺-ATPase (v-ATPase), and is the scaffold for amino acid-activated mTORC1 (mechanistic target of rapamycin complex 1), is critically required for M2 polarization. Lamtor1 deficiency, amino-acid starvation, or inhibition of v-ATPase and mTOR result in defective M2 polarization and enhanced M1 polarization. Furthermore, we identified liver X receptor (LXR) as the downstream target of Lamtor1 and mTORC1. Production of 25-hydroxycholesterol is dependent on Lamtor1 and mTORC1. Our findings demonstrate that Lamtor1 plays an essential role in M2 polarization, coupling immunity and metabolism.

Nur77-deficiency in bone marrow-derived macrophages modulates inflammatory responses, extracellular matrix homeostasis, phagocytosis and tolerance

Background

The nuclear orphan receptor Nur77 (NR4A1, TR3, or NGFI-B) has been shown to modulate the inflammatory response of macrophages. To further elucidate the role of Nur77 in macrophage physiology, we compared the transcriptome of bone marrow-derived macrophages (BMM) from wild-type (WT) and Nur77-knockout (KO) mice.

Results

In line with previous observations, SDF-1α (CXCL12) was among the most upregulated genes in Nur77-deficient BMM and we demonstrated that Nur77 binds directly to the SDF-1α promoter, resulting in inhibition of SDF-1α expression. The cytokine receptor CX3CR1 was strongly downregulated in Nur77-KO BMM, implying involvement of Nur77 in macrophage tolerance. Ingenuity pathway analyses (IPA) to identify canonical pathways regulation and gene set enrichment analyses (GSEA) revealed a potential role for Nur77 in extracellular matrix homeostasis. Nur77-deficiency increased the collagen content of macrophage extracellular matrix through enhanced expression of several collagen subtypes and diminished matrix metalloproteinase (MMP)-9 activity. IPA upstream regulator analyses discerned the small GTPase Rac1 as a novel regulator of Nur77-mediated gene expression. We identified an inhibitory feedback loop with increased Rac1 activity in Nur77-KO BMM, which may explain the augmented phagocytic activity of these cells. Finally, we predict multiple chronic inflammatory diseases to be influenced by macrophage Nur77 expression. GSEA and IPA associated Nur77 to osteoarthritis, chronic obstructive pulmonary disease, rheumatoid arthritis, psoriasis, and allergic airway inflammatory diseases.

Conclusions

Altogether these data identify Nur77 as a modulator of macrophage function and an interesting target to treat chronic inflammatory disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2469-9) contains supplementary material, which is available to authorized users.

Deficiency of the NR4A orphan nuclear receptor NOR1 in hematopoietic stem cells accelerates atherosclerosis

The NR4A orphan nuclear receptor NOR1 functions as a constitutively active transcription factor regulating cellular inflammation and proliferation. In this study, we used bone marrow transplantation to determine the selective contribution of NOR1 expression in hematopoietic stem cells to the development of atherosclerosis. Reconstitution of lethally irradiated apoE(-/-) mice with NOR1-deficient hematopoietic stem cells accelerated atherosclerosis formation and macrophage recruitment following feeding a diet enriched in saturated fat. NOR1 deficiency in hematopoietic stem cells induced splenomegaly and monocytosis, specifically the abundance of inflammatory Ly6C(+) monocytes. Bone marrow transplantation studies further confirmed that NOR1 suppresses the proliferation of macrophage and dendritic progenitor (MDP) cells. Expression analysis identified RUNX1, a critical regulator of hematopoietic stem cell expansion, as a target gene suppressed by NOR1 in MDP cells. Finally, in addition to inducing Ly6C(+) monocytosis, NOR1 deletion increased the replicative rate of lesional macrophages and induced local foam cell formation within the atherosclerotic plaque. Collectively, our studies demonstrate that NOR1 deletion in hematopoietic stem cells accelerates atherosclerosis formation by promoting myelopoiesis in the stem cell compartment and by inducing local proatherogenic activities in the macrophage, including lesional macrophage proliferation and foam cell formation.

Role of splenic monocytes in atherosclerosis

PURPOSE OF REVIEW

This review relates recent findings that highlight the role of the spleen as an active donor of monocytes during inflammation, with a special focus on atherosclerosis.

RECENT FINDINGS

The contribution of hypercholesterolemia and monocytes/macrophages to atherosclerotic lesion formation is undisputable. The origin of plaque macrophages is, however, still a subject of debate as to whether they derive from local amplification of (resident) macrophages or from continuous recruitment and differentiation of monocytes. Recently, the spleen has emerged as an important reservoir of monocytes that contributes to lesion growth. The regulation of monocyte mobilization from the splenic compartment has, therefore, raised a keen interest in understanding the cellular and molecular mechanisms involved in this process.

SUMMARY

Impaired regulation of cholesterol metabolism increases the proliferation of hematopoietic stem and progenitor cells in both the bone marrow and the spleen. Recent findings identified the implication of angiotensin II, red pulp macrophages and B-lymphocytes as partners of monocyte expansion in, and mobilization from the spleen. Future studies will help in understanding the mechanisms of monocyte mobilization and its precise roles in atherosclerosis, and whether modulation of the splenic components may become a promising future direction in the prevention and treatment of cardiovascular diseases.

Nur77 prevents excessive osteoclastogenesis by inducing ubiquitin ligase Cbl-b to mediate NFATc1 self-limitation

Osteoclasts are bone-resorbing cells essential for skeletal remodeling. However, over-active osteoclasts can cause bone-degenerative disorders. Therefore, the level of NFATc1, the master transcription factor of osteoclast, must be tightly controlled. Although the activation and amplification of NFATc1 have been extensively studied, how NFATc1 signaling is eventually resolved is unclear. Here, we uncover a novel and critical role of the orphan nuclear receptor Nur77 in mediating an NFATc1 self-limiting regulatory loop to prevent excessive osteoclastogenesis. Nur77 deletion leads to low bone mass owing to augmented osteoclast differentiation and bone resorption. Mechanistically, NFATc1 induces Nur77 expression at late stage of osteoclast differentiation; in turn, Nur77 transcriptionally up-regulates E3 ubiquitin ligase Cbl-b, which triggers NFATc1 protein degradation. These findings not only identify Nur77 as a key player in osteoprotection and a new therapeutic target for bone diseases, but also elucidate a previously unrecognized NFATc1→Nur77→Cblb—•NFATc1 feedback mechanism that confers NFATc1 signaling autoresolution.

DOI:http://dx.doi.org/10.7554/eLife.07217.001

Bones are constantly remodeled in response to the stresses of everyday life. Cells called osteoclasts break down old or damaged bone and cells called osteoblasts make new bone. In healthy bones, the work of these two types of cells is well balanced. But in bone-weakening diseases like osteoporosis and certain bone cancers this balance is disturbed and the osteoclasts become overly active, leading to weak and thin bones.

Some drugs can help block the development of osteoclasts and help reduce bone loss in these diseases, but they may cause unwanted side effects. A better understanding of the processes that maintain a healthy balance of osteoblasts and osteoclasts could help scientists develop better treatments with fewer side effects. Scientists have already learned that a protein called NFATc1 turns on the production of osteoclasts. But no one knew how NFATc1 is turned off in healthy bone to prevent the excessive growth of osteoclasts and too much bone turnover.

Now, Li et al. have identified a protein called Nur77 as an important regulator of NFATc1 by examining genetically engineered mice that lack Nur77. These modified mice had more osteoclasts and thinner bones than normal mice. Further experiments used radiation to wipe out the bone marrow of normal mice, who then received bone marrow transplants from mice that lacked Nur77. After the transplant, the normal mice showed bone loss. When the experiment was reversed, and Nur77-lacking mice received bone marrow from normal mice, their bone loss was alleviated. This indicates that Nur77 acts in the bone marrow cells to control osteoclasts and skeletal health.

Li et al. found that Nur77 cannot control the expression of the gene that encodes NFATc1 or directly bind to the NFATc1 protein. Instead, Nur77 increases the production of an enzyme that breaks down the NFATc1 protein. Unexpectedly, the experiments also found that NFATc1 turns on the expression of Nur77. This means that NFATc1 essentially regulates itself by increasing its own breakdown when NFATc1 levels increase. This helps to explain how osteoclast production is normally kept in check, and may suggest new strategies for treating bone diseases.

DOI:http://dx.doi.org/10.7554/eLife.07217.002

Foamy monocytes form early and contribute to nascent atherosclerosis in mice with hypercholesterolemia

OBJECTIVE

To examine infiltration of blood foamy monocytes, containing intracellular lipid droplets, into early atherosclerotic lesions and its contribution to development of nascent atherosclerosis.

APPROACH AND RESULTS

In apoE(-/-) mice fed Western high-fat diet (WD), >10% of circulating monocytes became foamy monocytes at 3 days on WD and >20% of monocytes at 1 week. Foamy monocytes also formed early in blood of Ldlr(-/-)Apobec1(-/-) (LDb) mice on WD. Based on CD11c and CD36, mouse monocytes were categorized as CD11c(-)CD36(-), CD11c(-)CD36(+), and CD11c(+)CD36(+). The majority of foamy monocytes were CD11c(+)CD36(+), whereas most nonfoamy monocytes were CD11c(-)CD36(-) or CD11c(-)CD36(+) in apoE(-/-) mice on WD. In wild-type mice, CD11c(+)CD36(+) and CD11c(-)CD36(+), but few CD11c(-)CD36(-), monocytes took up cholesteryl ester-rich very low-density lipoproteins (CE-VLDLs) isolated from apoE(-/-) mice on WD, and CE-VLDL uptake accelerated CD11c(-)CD36(+) to CD11c(+)CD36(+) monocyte differentiation. Ablation of CD36 decreased monocyte uptake of CE-VLDLs. Intravenous injection of DiI-CE-VLDLs in apoE(-/-) mice on WD specifically labeled CD11c(+)CD36(+) foamy monocytes, which infiltrated into nascent atherosclerotic lesions and became CD11c(+) cells that were selectively localized in atherosclerotic lesions. CD11c deficiency reduced foamy monocyte infiltration into atherosclerotic lesions. Specific and consistent depletion of foamy monocytes (for 3 weeks) by daily intravenous injections of low-dose clodrosome reduced development of nascent atherosclerosis.

CONCLUSIONS

Foamy monocytes, which form early in blood of mice with hypercholesterolemia, infiltrate into early atherosclerotic lesions in a CD11c-dependent manner and play crucial roles in nascent atherosclerosis development.

Macrophage phenotypes in atherosclerosis

Initiation and progression of atherosclerosis depend on local inflammation and accumulation of lipids in the vascular wall. Although many cells are involved in the development and progression of atherosclerosis, macrophages are fundamental contributors. For nearly a decade, the phenotypic heterogeneity and plasticity of macrophages has been studied. In atherosclerotic lesions, macrophages are submitted to a large variety of micro-environmental signals, such as oxidized lipids and cytokines, which influence the phenotypic polarization and activation of macrophages resulting in a dynamic plasticity. The macrophage phenotype spectrum is characterized, at the extremes, by the classical M1 macrophages induced by T-helper 1 (Th-1) cytokines and by the alternative M2 macrophages induced by Th-2 cytokines. M2 macrophages can be further classified into M2a, M2b, M2c, and M2d subtypes. More recently, additional plaque-specific macrophage phenotypes have been identified, termed as Mox, Mhem, and M4. Understanding the mechanisms and functional consequences of the phenotypic heterogeneity of macrophages will contribute to determine their potential role in lesion development and plaque stability. Furthermore, research on macrophage plasticity could lead to novel therapeutic approaches to counteract cardiovascular diseases such as atherosclerosis. The present review summarizes our current knowledge on macrophage subsets in atherosclerotic plaques and mechanism behind the modulation of the macrophage phenotype.

The neuron-derived orphan receptor 1 (NOR1) is induced upon human alternative macrophage polarization and stimulates the expression of markers of the M2 phenotype

BACKGROUND

Atherosclerosis is an inflammatory disease in which macrophages play a crucial role. Macrophages are present in different phenotypes, with at the extremes of the spectrum the classical M1 pro-inflammatory and the alternative M2 anti-inflammatory macrophages. The neuron-derived orphan receptor 1 (NOR1), together with Nur77 and Nurr1, are members of the NR4A orphan nuclear receptor family, expressed in human atherosclerotic lesion macrophages. However, the role of NOR1 in human macrophages has not been studied yet.

OBJECTIVES

To determine the expression and the functions of NOR1 in human alternative macrophages.

METHODS AND RESULTS

In vitro IL-4 polarization of primary monocytes into alternative M2 macrophages enhances NOR1 expression in human but not in mouse macrophages. Moreover, NOR1 expression is most abundant in CD68+MR+ alternative macrophage-enriched areas of human atherosclerotic plaques in vivo. Silencing NOR1 in human alternative macrophages decreases the expression of several M2 markers such as the Mannose Receptor (MR), Interleukin-1 Receptor antagonist (IL-1Ra), CD200 Receptor (CD200R), coagulation factor XIII A1 polypeptide (F13A1), Interleukin 10 (IL-10) and the Peroxisome Proliferator-Activated Receptor (PPAR)γ. Bioinformatical analysis identified F13A1, IL-1Ra, IL-10 and the Matrix Metalloproteinase-9 (MMP9) as potential target genes of NOR1 in human alternative macrophages. Moreover, expression and enzymatic activity of MMP9 are induced by silencing and repressed by NOR1 overexpression in M2 macrophages.

CONCLUSIONS

These data identify NOR1 as a transcription factor induced during alternative differentiation of human macrophages and demonstrate that NOR1 modifies the alternative macrophage phenotype.

Nonclassical patrolling monocyte function in the vasculature

Nonclassical patrolling monocytes are characterized by their unique ability to actively patrol the vascular endothelium under homeostatic and inflammatory conditions. Patrolling monocyte subsets (CX3CR1(high)Ly6C(-) in mouse and CX3CR1(high)CD14(dim)CD16(+) in humans) are distinct from the classical monocyte subsets (CCR2(high)Ly6C(+) in mouse and CCR2(high)CD14(+)CD16(-) in humans) and exhibit unique functions in the vasculature and inflammatory disease. Patrolling monocytes function in several disease settings to remove damaged cells and debris from the vasculature and have been associated with wound healing and the resolution of inflammation in damaged tissues. This review highlights the unique functions of these patrolling monocytes in the vasculature and during inflammation.

The orphan nuclear receptor Nur77 is a determinant of myofiber size and muscle mass in mice

We previously showed that the orphan nuclear receptor Nur77 (Nr4a1) plays an important role in the regulation of glucose homeostasis and oxidative metabolism in skeletal muscle. Here, we show using both gain- and loss-of-function models that Nur77 is also a regulator of muscle growth in mice. Transgenic expression of Nur77 in skeletal muscle in mice led to increases in myofiber size. Conversely, mice with global or muscle-specific deficiency in Nur77 exhibited reduced muscle mass and myofiber size. In contrast to Nur77 deficiency, deletion of the highly related nuclear receptor NOR1 (Nr4a3) had minimal effect on muscle mass and myofiber size. We further show that Nur77 mediates its effects on muscle size by orchestrating transcriptional programs that favor muscle growth, including the induction of insulin-like growth factor 1 (IGF1), as well as concomitant downregulation of growth-inhibitory genes, including myostatin, Fbxo32 (MAFbx), and Trim63 (MuRF1). Nur77-mediated increase in IGF1 led to activation of the Akt-mTOR-S6K cascade and the inhibition of FoxO3a activity. The dependence of Nur77 on IGF1 was recapitulated in primary myoblasts, establishing this as a cell-autonomous effect. Collectively, our findings identify Nur77 as a novel regulator of myofiber size and a potential transcriptional link between cellular metabolism and muscle growth.

Monocyte fate in atherosclerosis

Monocytes and their descendant macrophages are essential to the development and exacerbation of atherosclerosis, a lipid-driven inflammatory disease. Lipid-laden macrophages, known as foam cells, reside in early lesions and advanced atheromata. Our understanding of how monocytes accumulate in the growing lesion, differentiate, ingest lipids, and contribute to disease has advanced substantially over the last several years. These cells' remarkable phenotypic and functional complexity is a therapeutic opportunity: in the future, treatment and prevention of cardiovascular disease and its complications may involve specific targeting of atherogenic monocytes/macrophages and their products.

Mechanisms that regulate macrophage burden in atherosclerosis

Mononuclear phagocytes (MPs) relevant to atherosclerosis include monocytes, macrophages, and dendritic cells. A decade ago, studies on macrophage behavior in atherosclerotic lesions were often limited to quantification of total macrophage area in cross-sections of plaques. Although technological advances are still needed to examine plaque MP populations in an increasingly dynamic and informative manner, innovative methods to interrogate the biology of MPs in atherosclerotic plaques developed in the past few years point to several mechanisms that regulate the accumulation and function of MPs within plaques. Here, I review the evolution of atherosclerotic plaques with respect to changes in the MP compartment from the initiation of plaque to its progression and regression, discussing the roles that recruitment, proliferation, and retention of MPs play at these different disease stages. Additional work in the future will be needed to better distinguish macrophages and dendritic cells in plaque and to address some basic unknowns in the field, including just how cholesterol drives accumulation of macrophages in lesions to build plaques in the first place and how macrophages as major effectors of innate immunity work together with components of the adaptive immune response to drive atherosclerosis. Answers to these questions are sought with the goal in mind of reversing disease where it exists and preventing its development where it does not.

Macrophage phenotypes and their modulation in atherosclerosis

Atherosclerosis is the result of a chronic inflammatory response in the arterial wall related to uptake of low-density lipoprotein by macrophages and their subsequent transformation in foam cells. Monocyte-derived macrophages are the principal mediators of tissue homeostasis and repair, response to pathogens and inflammation. However, macrophages are a homogeneous cell population presenting a continuum phenotypic spectrum with, at the extremes, the classically Th-1 polarized M1 and alternatively Th-2 polarized M2 macrophage phenotypes, which have been well described. Moreover, M2 macrophages also present several subtypes often termed M2a, b, c and d, each of them expressing specific markers and exhibiting specialized properties. Macrophage plasticity is mirrored also in the atherosclerotic lesions, where different stimuli can influence the phenotype giving rise to a complex system of subpopulations, such as Mox, Mhem, M(Hb) and M4 macrophages. An abundant literature has described the potential modulators of the reciprocal skewing between pro-inflammatory M1 and anti-inflammatory M2 macrophages including lesion stage and localization, miRNA, transcription factors such as PPARγ, KLF4 and NR4A family members, high-density lipoproteins and plaque lipid content, pathways such as the rapamycin-mTOR1 pathway, molecules such as thioredoxin-1, infection by helminths and irradiation. We hope to provide an overview of the macrophage phenotype complexity in cardiovascular diseases, particularly atherosclerosis.

Targeting monocyte and macrophage subpopulations for immunotherapy: a patent review (2009 - 2013)

INTRODUCTION

Monocytes and macrophages are heterogeneous populations of effector cells in the innate immune system. Once thought to be obligatory precursors for macrophages, monocytes are now known to have several distinct sub-populations and their own independent functions. This separation of the two lineages has opened new therapeutic avenues in inflammation and created new technologies targeting the mononuclear phagocyte system (MPS).

AREAS COVERED

A search of Google Patents and PatentScope has revealed numerous patents targeting monocytes and macrophages. This review will focus on seven patents from 2009 to 2013, utilizing autologous monocyte and macrophage adoptive transfer, genetic manipulation of the MPS, therapeutic nanoparticles and liposomes or combinations of these strategies. Patents that target monocyte recruitment are also briefly reviewed.

EXPERT OPINION

While monocyte and macrophage targeting has yielded some promising results in animal models, these often fail to translate well to successful clinical trials. The paradigm of how cells in the MPS interact and evolve is constantly being updated, and caution must be exercised in developing immunomodulatory agents until this relationship is better understood.

The dynamic lives of macrophage and dendritic cell subsets in atherosclerosis

Atherosclerosis, the major pathological process through which arterial plaques are formed, is a dynamic chronic inflammatory disease of large- and medium-sized arteries in which the vasculature, lipid metabolism, and the immune system all play integral roles. Both the innate and adaptive immune systems are involved in the development and progression of atherosclerosis but myeloid cells represent the major component of the burgeoning atherosclerotic plaque. Various myeloid cells, including monocytes, macrophages (MΦs), and dendritic cells (DCs) can be found within the healthy and atherosclerotic arterial wall, where they can contribute to or regulate inflammation. However, the precise behaviors and functions of these cells in situ are still active areas of investigation that continue to yield exciting and surprising new data. Here, we review recent progress in understanding of the complex biology of MΦs and DCs, focusing particularly on the dynamic regulation of these subsets in the arterial wall and novel, emerging functions of these cells during atherogenesis.

Ly-6Chigh monocytes depend on Nr4a1 to balance both inflammatory and reparative phases in the infarcted myocardium

RATIONALE

Healing after myocardial infarction involves the biphasic accumulation of inflammatory lymphocyte antigen 6C (Ly-6C)(high) and reparative Ly-6C(low) monocytes/macrophages (Mo/MΦ). According to 1 model, Mo/MΦ heterogeneity in the heart originates in the blood and involves the sequential recruitment of distinct monocyte subsets that differentiate to distinct macrophages. Alternatively, heterogeneity may arise in tissue from 1 circulating subset via local macrophage differentiation and polarization. The orphan nuclear hormone receptor, nuclear receptor subfamily 4, group a, member 1 (Nr4a1), is essential to Ly-6C(low) monocyte production but dispensable to Ly-6C(low) macrophage differentiation; dependence on Nr4a1 can thus discriminate between systemic and local origins of macrophage heterogeneity.

OBJECTIVE

This study tested the role of Nr4a1 in myocardial infarction in the context of the 2 Mo/MΦ accumulation scenarios.

METHODS AND RESULTS

We show that Ly-6C(high) monocytes infiltrate the infarcted myocardium and, unlike Ly-6C(low) monocytes, differentiate to cardiac macrophages. In the early, inflammatory phase of acute myocardial ischemic injury, Ly-6C(high) monocytes accrue in response to a brief C-C chemokine ligand 2 burst. In the second, reparative phase, accumulated Ly-6C(high) monocytes give rise to reparative Ly-6C(low) F4/80(high) macrophages that proliferate locally. In the absence of Nr4a1, Ly-6C(high) monocytes express heightened levels of C-C chemokine receptor 2 on their surface, avidly infiltrate the myocardium, and differentiate to abnormally inflammatory macrophages, which results in defective healing and compromised heart function.

CONCLUSIONS

Ly-6C(high) monocytes orchestrate both inflammatory and reparative phases during myocardial infarction and depend on Nr4a1 to limit their influx and inflammatory cytokine expression.

From proliferation to proliferation: monocyte lineage comes full circle

Monocytes are mononuclear circulating phagocytes that originate in the bone marrow and give rise to macrophages in peripheral tissue. For decades, our understanding of monocyte lineage was bound to a stepwise model that favored an inverse relationship between cellular proliferation and differentiation. Sophisticated molecular and surgical cell tracking tools have transformed our thinking about monocyte topo-ontogeny and function. Here, we discuss how recent studies focusing on progenitor proliferation and differentiation, monocyte mobilization and recruitment, and macrophage differentiation and proliferation are reshaping knowledge of monocyte lineage in steady state and disease.

Limited role of nuclear receptor Nur77 in Escherichia coli-induced peritonitis

Nuclear receptor Nur77 (NR4A1, TR3, or NGFI-B) has been shown to play an anti-inflammatory role in macrophages, which have a crucial function in defense against peritonitis. The function of Nur77 in Escherichia coli-induced peritoneal sepsis has not yet been investigated. Wild-type and Nur77-knockout mice were inoculated with E. coli, and bacterial outgrowth, cell recruitment, cytokine profiles, and tissue damage were investigated. We found only a minor transient decrease in bacterial loads in lung and liver of Nur77-knockout compared to wild-type mice at 14 h postinfection, yet no changes were found in the peritoneal lavage fluid or blood. No differences in inflammatory cytokine levels or neutrophil/macrophage numbers were observed, and bacterial loads were equal in wild-type and Nur77-knockout mice at 20 h postinfection in all body compartments tested. Also, isolated peritoneal macrophages did not show any differences in cytokine expression patterns in response to E. coli. In endothelial cells, Nur77 strongly downregulated both protein and mRNA expression of claudin-5, VE-cadherin, occludin, ZO-1, and β-catenin, and accordingly, these genes were upregulated in lungs of Nur77-deficient mice. Functional permeability tests pointed toward a strong role for Nur77 in endothelial barrier function. Indeed, tissue damage in E. coli-induced peritonitis was notably modulated by Nur77; liver necrosis and plasma aspartate aminotransferase (ASAT)/alanine aminotransferase (ALAT) levels were lower in Nur77-knockout mice. These data suggest that Nur77 does not play a role in the host response to E. coli in the peritoneal and blood compartments. However, Nur77 does modulate bacterial influx into the organs via increased vascular permeability, thereby aggravating distant organ damage.

NR4A nuclear receptors in immunity and atherosclerosis

PURPOSE OF REVIEW

To understand chronic inflammatory diseases such as atherosclerosis, we require in-depth knowledge on immune-cell differentiation, function of specific immune-cell subsets and endothelial cell-mediated extravasation. In this review, we summarize a number of very recent observations on the pivotal function of NR4A nuclear receptors in immunity and atherosclerosis.

RECENT FINDINGS

NR4A nuclear receptors are involved in negative selection of thymocytes, Treg differentiation and the development of Ly6C monocytes. Nur77 and Nurr1 attenuate atherosclerosis in mice whereas NOR-1 aggravates vascular lesion formation.

SUMMARY

These exciting, novel insights on the function of NR4A nuclear receptors in immunity, vascular cells and atherosclerosis will initiate a plethora of studies to understand the underlying molecular mechanisms, which will culminate in the identification of novel NR4A targets to modulate chronic inflammatory disease.

Nuclear receptors expression chart in peripheral blood mononuclear cells identifies patients with Metabolic Syndrome

BACKGROUND

Nuclear receptors are a class of 48 ligand-activated transcription factors identified as key players of metabolic and developmental processes. Most of these receptors are potential targets for pharmacological strategies in the Metabolic Syndrome. In the present study, we analyzed changes in the mRNA expression of nuclear receptors in the peripheral blood mononuclear cells of patients with Metabolic Syndrome, in order to identify novel biomarkers of disease and candidate targets for putative therapeutical approaches.

METHODS AND RESULTS

We enrolled thirty healthy controls (14 M:16 F) and thirty naïve patients (16 M: 14 F; >3 criteria for Metabolic Syndrome upon Adult Treatment Panel III) without organ damage. Using quantitative real-time PCR, we assessed the expression patterns of nuclear receptors in peripheral blood mononuclear cells. 33/48 nuclear receptors were expressed in peripheral blood mononuclear cells. In patients with Metabolic Syndrome, we found a significant down-regulation of the entire PPAR, NR4A and RAR families, together with a repression of RXRα, VDR, and Rev-Erbα. Furthermore, we performed a novel statistical analysis with classification trees, which allowed us to depict a predictive core of nuclear receptor expression patterns characterizing subjects with Metabolic Syndrome. Random Forest Analysis identified NOR1 and PPARδ, which were both reduced in peripheral blood mononuclear cells and specifically in CD14(+) cells (mostly monocytes), as classifiers of Metabolic Syndrome, with high specificity and sensitivity.

CONCLUSIONS

Our results point to the use of PPAR and NR4A mRNA levels in the overall peripheral blood mononuclear cells as biomarkers of Metabolic Syndrome and bona fide putative targets of pharmacological therapy.

Macrophages in atherosclerosis: a dynamic balance

Atherosclerosis is a chronic inflammatory disease that arises from an imbalance in lipid metabolism and a maladaptive immune response driven by the accumulation of cholesterol-laden macrophages in the artery wall. Through the analysis of the progression and regression of atherosclerosis in animal models, there is a growing understanding that the balance of macrophages in the plaque is dynamic and that both macrophage numbers and the inflammatory phenotype influence plaque fate. In this Review, we summarize recently identified pro- and anti-inflammatory pathways that link lipid and inflammation biology with the retention of macrophages in plaques, as well as factors that have the potential to promote their egress from these sites.

Retinoid X receptors in macrophage biology

Retinoid X receptors (RXRs) form a distinct and unique subclass within the nuclear receptor (NR) superfamily of ligand-dependent transcription factors. RXRs regulate a plethora of genetic programs, including cell differentiation, the immune response, and lipid and glucose metabolism. Recent advances reveal that RXRs are important regulators of macrophages, key players in inflammatory and metabolic disorders. This review outlines the versatility of RXR action in the control of macrophage gene transcription through its heterodimerization with other NRs or through RXR homodimerization. We also highlight the potential of RXR-controlled transcriptional programs as targets for the treatment of pathologies associated with altered macrophage function, such as atherosclerosis, insulin resistance, autoimmunity, and neurodegeneration.